Why does my character model revert to a torso when I go into Pose mode to animate it?
The rigify addon creates an armature that uses drivers to control some parts of the rig. The leg bones have their scale calculated based on the value of the stretch_length, which is a custom property of the bone.

Blender has a security option that controls whether scripts are run automatically when opening a blend file, this includes drivers that use python expressions to define their calculations.

When you open a file with auto run scripts disabled these drivers aren't run so you end up with strange results like this.

The solution is to open the file with Trusted Source enabled or ensure it is in a path that is not listed as untrusted.